Hip joint disease is a common problem affecting large populations with multiple etiologies including osteoarthritis, avascular necrosis, post-traumatic arthritis and other hip joint pathologies. Over the course of the past century, many surgical techniques have been advocated in attempts to both provide pain relief and improve hip joint mechanics and patient function. Multiple techniques have been proposed and widely utilized over the years. Early techniques of joint replacement replaced the entire femoral head with a metallic implant, connected to a load-bearing femoral stem.
Hip Resurfacing Arthroplasty is a generally more conservative surgical technique that preserves more bone, including the femoral neck, since the surface of the femoral head is replaced using a hip resurfacing implant. The acetabulum is also resurfaced with implantation of an acetabular cup component. A common hip resurfacing femoral implant is illustrated as prior art in FIG. 1B.
Early Hip Resurfacing Arthoplasty is widely considered to have its roots in the work of John Charnley during the 1960's. More recently, Harlan Amstutz made further developments to hip resurfacing as illustrated in U.S. Pat. No. 4,123,806, showing a non-stemmed femoral hip resurfacing implant, and then in U.S. Pat. No. 6,156,069, showing a stemmed femoral hip resurfacing implant. A similar hip resurfacing system was developed by Derrick McMinn, an orthopedic surgeon from the West Midlands, UK. This system is known as the Birmingham Hip Resurfacing (BHR) system which uses a metal on metal design. Dr. McMinn subsequently developed the Birmingham Mid Head Resection (BMHR) implant, which is a modified version of the BHR system.
In all hip resurfacing systems, the femoral head needs to be prepared and shaped with a series of bone cuts. The Amstutz, BHR, and BMHR systems completely remove the dense, cortical bone of the femoral head leaving only the inner spongy, cancellous bone. In the Amstutz and BHR hip resurfacing implants, the upper, cap portion of the femoral implants seat onto the cancellous bone of the femoral head. The BMHR implant also does not utilize the femoral head cortex for implant support. This is a significant disadvantage of these implants as they do not gain additional peripheral stability from the strong and dense cortical bone of the femoral head. Bone preparation with the Amstutz, BHR, and BMHR systems include cylindrical reaming past the cartilage border, articular rim of the femoral head. This endangers the femoral neck as it may be notched during bone preparations from contact with the cylindrical reamer, which may then lead to the devastating complication of femoral neck fracture during subsequent load bearing.
Additionally, the Amstutz, BHR, and BMHR systems do not allow for additional threaded modular attachments for increasing fixation of the implant to the underlying bone. In the Amstutz, BHR, BMHR systems, bone cement is relied upon to secure the femoral implant to the underlying bone. While these implants are designed with porous surfaces allowing bony ingrowth, the implant is secured initially only with cement to the underlying bone. As the bony ingrowth takes time, bone cement is required for initial fixation of the device to the underlying bone of the femur. There are no additional fixation options for these implants in areas of poor bone quality. This is a another significant limitation as the strength of cancellous bone can be highly variable depending on local hip conditions including avascular necrosis, localized bone cysts and osteolysis. Soft cancellous bone can be encountered during surgery, but implant strength cannot be further increased with supplemental fixation. The BMHR system is designed to resect more of the cancellous bone of the femoral head, as compared to the BHR system, in order to attempt to address local bone deficiencies with use of an implant with a conical stem. However, this approach may remove healthy bone as a significant resection of femoral head bone is required during bone preparation for placement of the implant. The volume of bone resection required for the BMHR system is higher than that required for the BHR system. The Amstutz, BHR and BMHR designs also lack additional modular options for adding fixation. Thus, the Amstutz, BHR and BMHR hip resurfacing systems have limited flexibility to address bone deficiency.
Multiple surgical instruments are used to perform hip resurfacing arthroplasty. One such instrument is the alignment guide. A common example of an alignment guide used in hip resurfacing arthroplasty is the McMinn alignment guide from Smith & Nephew Orthopaedics Limited. Another such instrument is the cylindrical reamer, which is used to cut the bone of the femoral head, forming a cylindrical shape. A commonly used cylindrical reamer design for hip resurfacing arthroplasty is shown in U.S. Pat. No. 6,156,069. Following the cylindrical reaming procedure, a saw guide or cutoff guide is used to remove an additional small medial aspect of the bone of the femoral head.